A plane type heater has been conventionally manufactured using techniques for forming a thick film circuit such as screen printing as disclosed in, for example Japanese Patent Publication Gazette No. 55-24646. FIG. 3 is a sectional view showing a cathode of an electron tube using a conventional heater. In FIG. 3, reference numeral 10 designates a ceramic substrate, reference numeral 11 designates a heating element layer, reference numeral 12 designates an insulating layer, reference numeral 13 designates a cathode lead layer, reference numeral 14 designates a base metal layer and reference numeral 15 designates a cathode material layer.
Then, a manufacturing method therefor will be described in detail hereinafter.
First, a raw material for forming the ceramic substrate 10 is prepared and then the heating element layer 11 having a desired pattern is formed on a sheet by a printing technique such as an extrusion method by which a material is extruded between rollers or a casting method. The heating element layer 11 is formed on the ceramic substrate 10 by screen printing a paste in which baking assistant is added to a heater agent. Thereafter, it is baked at high temperature (1000.about.2000.degree. C.) and then the plane type heater is completed.
In this method, since there is a high temperature treatment in the manufacturing steps, when the heater is used at this temperature or less, it is expected that the heater will be stable at high temperature for a long time, for example a change of resistance with lapse of time is small. However, the pattern precision obtained by the screen printing is low and also it is difficult to control the thickness of the heating element 11, with the result that power consumption is large and the variation of resistance among a plurality of heaters is large. Therefore, a method for forming a film by PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) has been developed for forming a heater pattern with high precision.
FIG. 4 includes sectional views showing manufacturing steps for forming a conventional plane thin type heater by a thin film forming method. A resistor film for a heater 3 usually formed of metal such as tungsten is uniformly formed on a plane ceramic substrate as an insulating material a desired pattern is formed by etching, a lead wire (not shown) is connected thereto, and finally the plane type thin heater is completed.
However, when the plane type thin heater is manufactured by the above method, the adhesion between the resistor film 3 for the heater and the insulating material 1 is small. Therefore, in order to increase the adhesion between the resistor film 3 and the insulating material 1, an adhesive layer is used. Usually, a Ti film with a thickness of several tens to several hundreds of nanometers (nm) is formed as the adhesive layer and then the resistor layer is formed thereon to provide the thin high temperature heater. However, while the heater is used with a voltage applied thereto, that is, when a high temperature load of 1000.degree. C. is applied, Ti is degraded, causing the heater to break down. FIG. 6 shows the SEM photograph of a heater in a breakdown state (where 20.0KV is an accelerating voltage of the scanning electron microscope, .times.350 is a magnifying factor of 350, and a length of the line indicated corresponds to 100 microns). The reason for the breakdown is believed to be the transformation point of Ti from .alpha. to .beta. phase (referred to as a transformation point hereinafter) at 882.degree. C. and this transformation point is repetitively passed through while the heater is used. In addition, the resistance of the thin film resistor changes while it is used. FIG. 5 shows a change of resistance value with lapse of time. The reason why the resistance is reduced in an early stage is that recrystallization occurs in the thin film and then the size of the crystal grains in the thin film increase. For example, when the resistor (heating element) comprises W (tungsten) without an adhesive layer and is used at 1000.degree. C., since the temperature of 1000.degree. C. corresponds to the recrystallization temperature of W, recrystallization occurs. The reason why the resistance increases with lapse of time is that an impurity is mixed into the film or the film is oxidized by the ambient in use. In addition, since the insulating substrate of an oxide group such as Al.sub.2 O.sub.3 is available in its monocrystalline state and a surface thereof can be mirror-finished, its pattern precision thereof is better than sintered substrates such as SiC, and AlN. Therefore, Al.sub.2 O.sub.3 has been conventionally used as the insulating substrate (insulating material) of the conventional thin film high temperature heater manufactured by the thin film forming method. However, in the conventional heater using Al.sub.2 O.sub.3, when the heating element is directly formed on the insulating material, the insulating material reacts with the heating element by thermochemical or electrochemical action caused by oxygen. Therefore, a substance which is likely to sublime is formed. As a result, an edge at which the Al.sub.2 O.sub.3 substrate and the heating element such as W are both in contact with ambient in the vicinity of resistance wiring end (heating element end) is selectively damaged. FIGS. 7 and 8 are scanning electron microscope photographs showing two ends of a damaged heater. Thus, the conventional thin high temperature heater formed by the thin film forming methods is unstable as a heater and also unreliable as far as a long term use is concerned.